Method and system to determine physical parameters as between a RFID tag and a reader

ABSTRACT

A method and system to determine physical parameters as between a RFID tag and a reader. At least some of the illustrative embodiments are methods comprising generating an antenna feed signal, and transmitting a first electromagnetic wave to a radio frequency device (by coupling the antenna feed signal to a reading antenna), receiving a backscattered electromagnetic wave from the radio frequency device to create a received signal, calculating a combined signal based on the antenna feed signal and received signal, and determining relative velocity between the radio frequency device and the reading antenna based on the combined signal.

BACKGROUND

1. Field

The various embodiments are directed to determining physical parameters(e.g. velocity and acceleration) as between objects tagged with radiofrequency identification (RFID) tags and reader circuits.

2. Description of the Related Art

Radio frequency identification (RFID) tags are used in a variety ofapplications, such as tagging vehicles on toll roads, tagging shippingcontainers, quality control on assembly line conveyer belts, andmonitoring tactical military equipment maneuvers. In many situations itwould be valuable to know physical parameters of the RFID tags and/orthe objects coupled to the tags.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various embodiments, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows a radio frequency identification (RFID) system inaccordance with at least some embodiments;

FIG. 2 shows a system in accordance with at least some embodiments;

FIG. 3 shows calculation of sum signal and envelope signal;

FIG. 4 shows a system in accordance with other embodiments;

FIGS. 5A and 5B show multi-path signals and their envelope; and

FIG. 6 shows a method in accordance with at least some embodiments.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, design and manufacturing companies may refer to a componentby different names. This document does not intend to distinguish betweencomponents that differ in name but not function.

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . ” Also, theterm “couple” or “couples” is intended to mean either an indirect ordirect connection. Thus, if a first device couples to a second device,that connection may be through a direct connection or through anindirect connection via other intermediate devices and connections.Moreover, the term “system” means “one or more components” combinedtogether. Thus, a system can comprise an “entire system,” “subsystems”within the system, a radio frequency identification (RFID) tag, a readercircuit, or any other device comprising one or more components.

DETAILED DESCRIPTION

The various embodiments disclosed herein are discussed in the context ofradio frequency identification (RFID) tags; however, the systems andmethods discussed have application beyond RFID tags to other types ofradio frequency technologies. The discussion of any embodiment inrelation to RFID tags is meant only to be illustrative of thatembodiment, and not intended to intimate that the scope of thedisclosure, including the claims, is limited to that embodiment.

FIG. 1 shows a system 100 in accordance with some embodiments. Inparticular, system 100 comprises an electronic system 21 (e.g., acomputer system) coupled to a radio frequency identification (RFID)reader circuit 19. The reader circuit 19 may be equivalently referred toas an interrogator. By way of antenna 18, the reader circuit 19communicates with one or more RFID tags 16A-16C proximate to the readercircuit (i.e., within communication range). In particular, the readercircuit 19 transmits an interrogating electromagnetic wave tocommunicate with one or more of the RFID tags 16A-16C.

Considering a single RFID tag 16A (but the description equallyapplicable to all the RFID tags 16), the communication sent by thereader circuit 19 is received by tag antenna 17A, and passed to the RFIDcircuit 15A. If the communication from the reader circuit triggers aresponse, the RFID circuit 15A sends to the reader circuit 19 theresponse (e.g., a tag identification value, or data held in the tagmemory) using the tag antenna 17A. The reader circuit 19 passes dataobtained from the various RFID tags 16 to the electronic system 21,which performs any suitable function. For example, the electronic system21, based on the data received from the RFID tags 16, may allow accessto a building or parking garage, note the entrance of an employee to awork location, direct a parcel identified by the RFID tag 16 down aparticular conveyor system, or inventory products in a shopping cart forpurposes of checkout and payment. In accordance with some embodiments,the reader circuit 19 and/or the electronic system 21 also determinephysical parameters as between the RFID tag 16A using, at least in part,backscattered electromagnetic waves from the RFID tag 16A. Thus, thediscussion turns to a description of backscattered electromagnetic wavesproduced by RFID tags.

There are several types of RFID tags operable in the illustrative system100 that produce backscattered electromagnetic waves. For example, RFIDtags may be semi-active tags, meaning each RFID tag comprises its owninternal battery or other power source, but a semi-active tag remainsdormant (i.e., powered-off or in a low power state) most of the time.When an antenna of a semi-active tag receives an interrogatingelectromagnetic wave, the power received is used to wake or activate thesemi-active tag, and a response (if any) comprising an identificationvalue is sent by modulating the backscattered electromagnetic wave fromthe tag antenna, with the semi-active tag using power for internaloperations from its internal battery or power source. In particular, thereader circuit 19 and antenna 18 continue to transmit power after theRFID tag is awake. While the reader circuit 19 transmits, the tagantenna 17 of the RFID tag 16 is selectively tuned and de-tuned withrespect to the carrier frequency. When tuned, significant incident poweris absorbed by the tag antenna 17. When de-tuned, significant power isreflected or backscattered by the tag antenna 17 to the antenna 18 ofthe reader circuit 19.

A second type of RFID tag that produces backscattered electromagneticwaves is a passive tag, which, unlike semi-active RFID tags, has nointernal battery or power source. The tag antenna 17 of the passive RFIDtag receives an interrogating electromagnetic wave from the readercircuit, and the power extracted from the received interrogatingelectromagnetic wave is used to power the tag. Once powered or “awake,”the passive RFID tag may accept a command, send a response comprising adata or identification value, or both; however, like the semi-active tagthe passive tag sends the response in the form of a backscatteredelectromagnetic wave.

In accordance with at least some embodiments, the reader circuit 19and/or the electronic system 21 determine physical parameters (e.g.velocity) between the RFID tag 16 and the reading antenna 18 using, atleast in part, backscattered electromagnetic waves. The backscatteredelectromagnetic waves received at the reading antenna 18 from the tagantenna 17 can be used coherently in combination with the interrogatingwave transmitted from the reading antenna 18 to the tag antenna 17.Thus, the discussion now turns to determining physical parameters in aRFID system using backscattered electromagnetic waves.

In accordance with the various embodiments, the electronic system 21and/or the reader circuit 19 generates an antenna feed signal at acarrier frequency (e.g. using a local oscillator, or using a digitalsignal processor by way of a digital to analog converter). The termantenna feed signal is not limited to just high voltage/high currentsignal directly applied to an antenna, and also refers to the initialsignal before voltage/current amplification. The antenna feed signal isapplied to the reading antenna 18, which in turn transmits anelectromagnetic wave to interrogate the RFID tag 16 (hence, aninterrogating electromagnetic wave). The RFID tag 16 (i.e. semi-activetag or passive tag) responds with a backscattered electromagnetic wave,as discussed above. The reading antenna 18 converts incidentbackscattered electromagnetic wave into a received signal havingfrequency, phase and amplitude corresponding to the backscatteredelectromagnetic wave. The received signal is then passed to the readercircuit 19 and/or electronic system 21. The reader circuit 19 and/orelectronic system 21 combine the antenna feed signal and receivedsignal, and analyze the combined signal to determine the physicalparameters.

FIG. 2 shows a system 200 in accordance with some embodiments. Inparticular, system 200 shows an object 10 on a conveyor system 12, withthe object 10 selectively moving in the direction indicated by arrow 14.Conveyor system 12 is merely illustrative of any situation where anobject 10 moves in two- or three-dimensional space. For example, theobject 10 and conveyor system 12 are illustrative of wafer boats insemiconductor manufacturing production line, luggage in an automatedluggage handling system, parcels in an automated sorting facility, orparticipants in a war game. The object 10 has an associated RFID tag 16,which as illustrated is visible from both sides of object 10. In system200, the reading antenna 18 is shown as a Yagi-Uda antenna, but otherantenna types (e.g., dipole, loop or patch antennas) may be equivalentlyused. FIG. 2 further shows three illustrative positions of the RFID tag16 and/or object 10. In particular, FIG. 2 shows an initial location31A, and two subsequent locations 31B and 31C. For reasons that willbecome apparent in reference to FIG. 3, the illustrative physicaldistance between the initial location 31A and location 31B is half awavelength of the interrogating electromagnetic wave, and theillustrative physical distance between the initial location 31A andlocation 31C (i.e., distance 35) is one wavelength of the interrogatingelectromagnetic wave.

FIG. 3 illustrates various time varying signals associated with sendingthe interrogating electromagnetic wave and receiving backscatteredelectromagnetic waves to more fully explain the signals and how thesignals are combined to determine various parameters. In particular,FIG. 3 illustrates an antenna feed signal 30 which is applied to thereading antenna 18 during interrogation. The RFID tag 16 (which isassumed for this example to be moving at a constant velocity) reflects aportion of the interrogating electromagnetic wave to create reflected orbackscattered electromagnetic wave that is incident upon the readingantenna 18, and therefore creates a received signal 32, illustrated inFIG. 3 as received signal portions 32A-32C (optionally scaled byamplicification). Referring simultaneously to FIGS. 2 and 3, each of thereceived signal portions 32A, 32B and 32C are based on the backscatteredelectromagnetic wave received from the RFID tag 16 as the tag movesthrough positions 31A, 31B and 31C, respectively. At least because ofthe difference in distance between the RFID tag 16 and the readingantenna 18 for each location 31A, 31B and 30C, each of the receivedsignal portions 32A-32C has an associated phase and amplitude that maybe different from the other portions, and may also be different fromantenna feed signal 30. FIG. 3 illustrates only three portions ofreceived signal 32 corresponding to three particular positions of theRFID tag 16 so as to not to unduly complicate the drawings; however, inactuality one continuous backscattered electromagnetic wave is receivedby the reading antenna 18 producing one continuous received signal 32,and the received signal 32 has varying amplitude and phase based on thedistance from the RFID tag to the reading antenna and the velocity atwhich the RFID tag is moving relative to the reading antenna.

In order to determine physical parameters as between the RFID tag andthe reading antenna, and in accordance with at least some embodiments,the reader circuit 19 and/or electronic system 21 combine the antennafeed signal 30 and the received signal 32 over a period of time togenerate a combined signal 36. Combining the antenna feed signal 30 andthe received signal 32 may take many forms. In some embodiments, theantenna feed signal 32 and received signal are summed in analog form tocreate combined signal 36. In yet still other embodiments, the antennafeed signal 30 and received signal 32 are mixed (i.e., multiplied) inanalog form to create the combined signal 36. In yet still otherembodiments, the antenna feed signal 30 and received signal 32 may becombined (e.g. summed, mixed) in digital form. Other digital techniquesmay comprise exclusive-ORing (XOR) portions or all of the digitalrepresentation of the antenna feed signal 30 and received signal 32.

Regardless of whether performed using analog or digital versions of thesignals, and regardless of the precise form of the combining, in some ofthe embodiments before the received signal 32 and the antenna feedsignal 30 are combined the reader circuit 19 and/or electronic system 21normalize the amplitude of the signals. Normalization is performed toavoid combining signals that have peak amplitude substantially differentform each other. In FIG. 3 the combined signal 36 is logically dividedinto smaller portions 34A-34C for purposes of discussion. Consideringfirst portion 34A of the combined signal 36, portion 34A is thecombination (in the illustrative case of FIG. 3, a sum) of portion 32Aof the received signal with corresponding portions of the antenna feedsignal 30. Since in this illustrative example the two signals are 180degrees out of phase with each other, the signals cancel producingzero-value or null portion of the sum signal 36 (a similar result occurswhen the signals are combined by mixing). Similarly, third portion 34Cof the combined signal 36 is the combination of portion 32C of thereceived signal with corresponding portions of the antenna feed signal30. Here again, since in this illustrative example the two signals are180 degrees out of phase with each, the signals cancel producing azero-value portion 34C of the combined signal 36. Finally, portion 34Bof the combined signal 36 is the combination of portion 32B of thereceived signal with corresponding portions of the antenna feed signal30. Since in this illustrative example the signals are in phase witheach other, the portion 34B of the sum signal 36 has amplitude that isapproximately two times that of the signals considered alone.

The final combined signal 36 has varying amplitude due the phasedifference in the antenna feed signal 30 and the received signal 32, anddefines an envelope 37. The reader circuit 19 and/or electronic system21 determine the envelope from the combined signal 36 (e.g. using ademodulator) to produce envelope signal 38. Illustrative envelope signal38 comprises a plurality of inflection points 39A-39C based on changingphysical distance between the RFID tag 16 and the reading antenna 18(e.g., as shown in FIG. 2). In particular, in the illustrations of FIGS.2 and 3 the inflection points 39A-39C correlate with the positions31A-30C of the object 10 on the conveyer system 12. The correlationexists because at some physical distances the phase of the receivedsignal (corresponding to the backscattered electromagnetic wave) and theantenna feed signal (corresponding to the interrogating electromagneticwave) cancel when combined, and at other physical distances the phasesare aligned. In accordance with various embodiments, the reader circuit19 and/or electronic system 21 find at least some inflection points(e.g., maxima, minima, nulls, or zero-points) in the envelope signal 38.Using at least some of the infection points, the reader circuit 19and/or electronic system 21 determine physical parameters as betweenRFID tag and the reading antenna.

Consider, for purposes of explanation, that the reader circuit 19 and/orthe electronic system 21 find and use inflection points being nulls inthe envelope signal 38 as the mechanism to determine physical parametersas between the RFID tag 16 and reading antenna 18. As discussed above,the illustrative positions 31A and 31C of the RFID tag 16 correspond tonulls 39A and 39C of the envelope signal 38, respectively. Further, thedistance between the nulls, distance 35 in FIG. 2, was defined to be onewavelength of the interrogating electromagnetic wave. In some of theembodiments then, the velocity of the RFID tag 16 is calculatedaccording to the following equation:

$\begin{matrix}{V_{tag} = \frac{\Delta\;{NullCounts}*\lambda}{\Delta\;{Seconds}}} & (1)\end{matrix}$where V_(tag) is the velocity of the RFID tag, ΔSeconds is a period oftime in seconds between the first and last nulls, ΔNullCounts is thenumber of nulls occurring over ΔSeconds, and λ is the wavelength of theinterrogating electromagnetic wave. The remaining portions of thisdescription are based on determining inflection points being nulls inthe envelope signal 38; however, using nulls as the inflection points ismerely illustrative. Any corresponding inflection points within theenvelope signal 38 (e.g., maxima points, minima points) may be used todetermine V_(tag), and thus ΔNullCounts in equation (1) may begeneralized to be ΔCorrespondingInflectionPoints. To provide bestaccuracy, if the envelope is sampled such that the sampled dated beginsbefore a first ΔCorrespondingInflectionPoints and/or extends beyond alater occurring final ΔCorrespondingInflectionPoints, the ΔSeconds datamay be determined using only the time between the first and finalΔCorrespondingInflectionPoints, ignoring the time before the firstΔCorrespondingInflectionPoints and the time after the finalΔCorrespondingInflectionPoints.

As a numerical example, consider that the interrogating electromagneticwave has a frequency of 900 Mega-Hertz (MHz), which corresponds to awavelength λ of approximately 0.3 meters or approximately 12 inches. Iftwo nulls are found in the envelope signal 38 over a predeterminedperiod of one second, then V_(tag) for RFID tag would be 0.6meters/second or 24 inches/second.

Determining velocity of the RFID 16 based on counting the number ofnulls over a period of time may be extended to determine position. Forexample, if a starting position is known or estimated (e.g. using amechanical trigger, optical trigger and/or a pre-defined initial value),a new position can be determined using the nulls. In some of theembodiments, a new position of the RFID tag 16 is calculated accordingto the following equation:NewPosition=StartPosition±(ΔNullCounts*λ*ΔSeconds)  (2)where NewPosition is the calculated new position, StartPosition is theknown or estimated starting position, λ and ΔSeconds are as discussedwith respect to equation (1), and whether to add or subtract theparenthetical term is based on whether the RFID tag 16 and underlyingobject are move toward or away from the reading antenna and where thetag velocity is constant within an acceptable tolerance. With respect todetermining whether the RFID tag is moving toward or away from thereading antenna 18, the reader circuit 19 and/or electronic system 21may observe the return signal strength indication (RSSI) of the receivedsignal 32 (before normalization), and add or subtract the parentheticalterm of equation (2) based on RSSI. For example, an increasing RSSI isindicative of the RFID tag moving toward the reading antenna (or viceversa), thus indicating subtraction of the parenthetical term.Conversely, a decreasing RSSI is indicative of the RFID tag moving awayfrom the reading antenna (or vice versa), thus indicating addition ofthe parenthetical term.

Using the illustrative numerical example above of a 900 MHzinterrogating electromagnetic wave, two null counts in one second, anassumed starting position of 12 inches from the reading antenna and anassumed direction of the RFID tag moving away from the reading antenna,the new position of the RFID tag is calculated to be 12″+(2*12″)=36″from the reading antenna. For best accuracy, if time data for thepurpose of determining V_(tag) uses only the time between first andfinal ΔCorrespondingInflectionPoints, thereby ignoring time before thefirst ΔCorrespondingInflectionPoint and after the finalΔCorrespondingInflectionPoint, then the ignored time may be added on todetermine that total ΔSeconds for position.

The various embodiments, however, are not limited to determining justvelocity and/or location, as other parameters that are based on velocitymay also be determined. For example, acceleration is the first timederivative of V_(tag), and jerk (equivalently referred to as jolt) isthe second time derivative of V_(tag).

Returning to FIG. 2, in accordance with other embodiments the system 200has a second reader circuit 23 and reading antenna 20 placed, forexample, on the opposite side as the first reader circuit 19 and readingantenna 18. In such embodiments, any physical parameter (e.g., velocity,position, acceleration and jerk) may be verified by signals from readingantenna 20. In other embodiments, the second reader circuit 23 andreading antenna 20 are placed such that they are at an angle with eachother, (e.g. a right angle). In the yet still other embodiments, a thirdreader is placed orthogonal to the plane established by the reader 19and reader 23.

FIG. 4 illustrates other embodiments where a single reader circuit 19couples to multiple reading antennas. In particular, FIG. 4 shows tworeading antennas 104A and 104B (which antennas could be, for example,antennas 18 and 20 respectively of FIG. 3). The reading antennas 104Aand 104B in FIG. 4 couple to a single reader circuit 19 by way ofmultiplexer 106, and the reader circuit 19 also couples to an electronicsystem 21.

Many atmospheric conditions and/or man-made objects affectelectromagnetic wave propagation, and thus lead to signal degradation.Multi-path degradation is a type of signal degradation where multiplebackscattered electromagnetic waves arrive at the reading antenna by wayof different paths. FIG. 5A illustrates such degradation. In particular,reading antenna 18 coupled to the reader circuit 19 receivesbackscattered electromagnetic waves from the RFID tag 16 associated withan object 10. The backscattered electromagnetic waves in FIG. 5Apropagate using multiple paths to reach the reading antenna 18. In theillustration, some of the backscattered electromagnetic waves propagatealong a line-of-sight path 50, and some of the backscatteredelectromagnetic waves are reflected due atmospheric conditions and/orman-made objects 52 and propagate along a different path 54 beforereaching the reading antenna. Convergence of the multi-pathbackscattered waves at the reading antenna 18 has unfavorableconsequences.

FIG. 5B illustrates two different envelope signals 56 and 58 calculatedby the reader circuit 19 and/or electronic system 21. The envelopesignal 56 is similar to envelope signal 38 (FIG. 2), and is asubstantially smooth signal with three inflection points 51A-51C. Theenvelope signal 58 is not a substantially smooth signal, and hasinflection points 51A-51C, along with extraneous inflection points53A-53D. The signal 56 is associated with backscattered electromagneticwaves which do not propagate along multiple paths, whereas the signal 58is associated with backscattered electromagnetic waves propagating alongmultiple paths 50 and 54 (as shown in FIG. 5A). In some embodiments, themulti-path degradation of the received backscattered electromagneticwaves is reduced by ignoring extraneous inflection points 53A-53D. Inparticular, the inflection points in envelope signals are classifiedinto categories (e.g. inflection points associated with line-of-sightwaveforms and inflection points associated with multi-path waveforms).The inflection points associated with multi-path waveforms are ignoredand inflection points associated with line-of-sight waveforms are usedin calculating the physical parameters.

In some of the embodiments, the RFID tag 16 responds to theinterrogating electromagnetic wave with a tag identification value, ordata held in the tag memory. In these embodiments, determining physicalparameters as between the RFID tag and the reading antenna are based onperiods of time when the RFID tag is reflective. However, in somesituations the periods of time when the RFID tag is reflective as partof communicating data may be insufficient to determine the physicalparameters (i.e., the data rate is too high and the reflective period istherefore too short). Thus, in other embodiments RFID tag 16 may beplaced in a reflective mode such that, for extended periods of timerelative to selectively backscattering to send data, the RFID tag is ina purely reflective mode. The reader circuit 19 and/or electronic system21 in these embodiments are configure to send a command instructing theRFID tag 16 to change its operation to a constant or alternatingrepetitive state; hence, the electromagnetic waves received at thereading antenna 18 are only backscattered electromagnetic wave withoutany associated data. The RFID tag 16 is configured to time out of thereflective state, or the command sent to place the tag in the reflectivestate, may comprise a period of time for the RFID tag to stayreflective, and then revert to prior operational modes. Thus, by settingthe RFID tag 16 to a reflective state determining of physical parametersas between the RFID tag 16 and reading antenna 18 can occur for a longerperiods of time, or occur more rapidly.

FIG. 6 shows a method in accordance with at least some embodiments. Inparticular, the method starts (block 600) and moves to generating anantenna feed signal (block 604). Next, an interrogating electromagneticwave is sent to a radio frequency device (block 608), where theinterrogating electromagnetic wave is based on the antenna feed signal.In some embodiments, the radio frequency device is a RFID tag.Thereafter, a backscattered electromagnetic wave is received from theradio frequency device to create a received signal (block 612). Next, acombined signal is calculated based on the antenna feed signal andreceived signal (block 616). Finally, a determination is made as to therelative velocity between the radio frequency device and the readingantenna based on the combined signal (block 620), and the method ends(block 624).

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. For example, the combining of theantenna feed signal and received signal may be performed with thosesignal in analog form, or the combining may be performed with digitalrepresentations of the signals. Moreover, the various embodiments arediscussed with respect to the RFID tag moving and the reader circuitstationary; however, in other embodiments the physical parameters aredetermined with respect to a stationary RFID tag and reader circuitmoving (e.g. in a warehouse in order to inventory objects on shelves).Finally, while in the various embodiments discussed the interrogatingelectromagnetic wave is transmitted from the same antenna as receivesthe backscattered electromagnetic wave, in alternative embodimentsseparate antennas may be used with one to transmit the interrogatingelectromagnetic wave, and a the second to receive the backscatteredelectromagnetic wave. It is intended that the following claims beinterpreted to embrace all such variations and modifications.

1. A method comprising: generating an antenna feed signal; transmittinga first electromagnetic wave to a radio frequency device by coupling theantenna feed signal to a reading antenna; receiving a backscatteredelectromagnetic wave from the radio frequency device to create areceived signal; calculating a combined signal based on the antenna feedsignal and received signal, wherein the combined signal includes aplurality of inflection points, each inflection point corresponding to adifferent position of the radio frequency device relative to the readingantenna, wherein the plurality of inflection points comprises a firstinflection point, a last inflection point, and at least one intermediateinflection point between the first inflection point and the lastinflection point; and determining a relative velocity between the radiofrequency device and the reading antenna by detecting the inflectionpoints of the combined signal over a predetermined time period, whereinthe predetermined time period begins at a time associated with the firstinflection point and ends at a time associated with the last inflectionpoint.
 2. The method as defined in claim 1 wherein the radio frequencydevice is a radio frequency identification (RFID) tag.
 3. The method asdefined in claim 1 wherein calculating further comprises calculating thecombined signal by at least one selected from the group consisting of:summing the antenna feed signal and the received signal; mixing theantenna feed signal with the received signal; performing an exclusive-ORusing at least portions of the antenna feed signal and the receivedsignal.
 4. The method as defined in claim 1 wherein the inflectionpoints comprise at least one selected from the group consisting of: anull; minima; and maxima.
 5. The method as defined in claim 1 whereinthe determining the relative velocity further comprises at least oneselected from the group consisting of: determining the velocity of theradio frequency device relative to a moving reading antenna; anddetermining the velocity of the reading antenna relative to a movingradio frequency device.
 6. The method of claim 1, wherein the firstinflection point and the last inflection point each correspond to a nullvalue.
 7. A system comprising: a radio frequency identification (RFID)tag; a reading antenna in operational relationship to the RFID tag; anda reader circuit coupled to the reading antenna, the reader circuitconfigured to transmit a first electromagnetic wave to the RFID tag, thefirst electromagnetic wave based on an antenna feed signal; wherein thereader circuit is configured to receive a received signal, the receivedsignal being based on a backscattered electromagnetic wave received fromthe RFID tag; wherein the reader circuit generates a combined signal ofthe antenna feed signal and the received signal; wherein the combinedsignal includes a plurality of inflection points, each inflection pointcorresponding to a different position of the RFID tag relative to thereading antenna; wherein the plurality of inflection points comprises afirst inflection point, a last inflection point, and at least oneintermediate inflection point between the first inflection point and thelast inflection point; and wherein the reader circuit determines arelative velocity between the RFID tag and the reading antenna bydetecting the inflection points of the combined signal over apredetermined time period, wherein the predetermined time period beginsat a time associated with the first inflection point and ends at a timeassociated with the last inflection point.
 8. The system as defined inclaim 7 wherein the reader circuit determines the relative velocity bycounting the inflection points of the combined signal.
 9. The system asdefined in claim 8 wherein the inflection points comprise at least oneselected from the group consisting of: nulls; maxima; and minima. 10.The system as defined in claim 7 wherein the system is configured tocreate the combined signal by at least one selected from the groupconsisting of: a sum of the antenna feed signal and the received signal;a multiplication of the antenna feed signal with the received signal;performance of an exclusive—OR using at least portions of the antennafeed signal and the received signal.
 11. The system as defined in claim7 wherein the system is further configured to determine at least oneselected from the group consisting of: relative position; acceleration;and jerk.
 12. The system as defined in claim 7 wherein the RFID tagfurther comprises at least one selected from the group consisting of: apassive tag; and a semi-active tag.
 13. A radio frequency identification(RFID) reader comprising: a reading antenna configured to receivebackscattered electromagnetic waves from a RFID tag to create a receivedsignal; a reader circuit coupled to the reading antenna, the readercircuit being configured to transmit an interrogating electromagneticwave to the RFID tag, wherein the interrogating electromagnetic wave isbased on an antenna feed signal; wherein the reader circuit isconfigured to determine a relative velocity between the RFID tag and thereading antenna based on a combined signal of the antenna feed signaland the received signal, wherein the combined signal includes aplurality of inflection points, each inflection point corresponding to adifferent position of the RFID tag relative to the reading antenna,wherein the plurality of inflection points comprises a first inflectionpoint, a last inflection point, and at least one intermediate inflectionpoint between the first inflection point and the last inflection point,wherein the reader circuit determines the relative velocity by detectingthe inflection points of the combined signal over a predetermined timeperiod, wherein the predetermined time period begins at a timeassociated with the first inflection point and ends at a time associatedwith the last inflection point.
 14. The radio frequency identificationreader as defined in claim 13 wherein the inflection points comprise atleast one selected from the group consisting of: nulls; maxima; andminima.
 15. The radio frequency identification reader as defined inclaim 13 wherein the reader circuit is configured to determine at leastone selected from the group consisting of: relative position;acceleration; and jerk.
 16. A non-transitory computer-readable mediumstoring a program that, when executed by a processor, causes theprocessor to: cause an antenna feed signal to be applied to a readingantenna to create an interrogating electromagnetic wave; receive areceived signal based on a backscattered electromagnetic wave receivedfrom a RFID tag; calculate a combination signal being the combination ofthe antenna feed signal and the received signal, wherein the combinationsignal includes a plurality of inflection points, each inflection pointcorresponding to a different position of the RFID tag relative to theRFID reader, wherein the plurality of inflection points comprises afirst inflection point, a last inflection point, and at least oneintermediate inflection point between the first inflection point and thelast inflection point; determine a relative velocity between the RFIDtag and the reading antenna by detecting the inflection points of thecombined signal over a predetermined time period, wherein thepredetermined time period begins at a time associated with the firstinflection point and ends at a time associated with the last inflectionpoint.
 17. The non-transitory computer-readable medium as defined inclaim 16 wherein the inflection points comprise at least one selectedfrom the group consisting of: nulls; minima; and maxima.
 18. Thenon-transitory computer-readable medium as defined in claim 16 whereinwhen the processor determines the relative velocity, the program causesthe processor to determine at least one selected from the groupconsisting of: the velocity of the RFID tag relative to a moving readingantenna; and the velocity of the reading antenna relative to a movingRFID tag.
 19. The non-transitory computer-readable medium as defined inclaim 16 wherein when the processor determines, the program causes theprocessor to determine at least one selected from the group consistingof: relative position; acceleration; and jerk.